The present invention relates to the field of organic catalytic synthesis. More particularly, it concerns a method for stereospecific isomerisation of allylamines, such as those defined by formula (I) below, using complexes of certain transition metals having, as ligands, chiral phosphorated compounds immobilised by fixation on to a suitable polymer.
Isomerisation reactions of prochiral and non-prochiral allylamines, with the aid of complexes of rhodium, iridium or ruthenium have already been known for several years; examples are those represented by the formulae [Rh(Pxe2x80x94P)*diene]+Xxe2x88x92, [Rh(Pxe2x80x94P)*2]+Xxe2x88x92, [Ru(CH3COO)2(Pxe2x80x94P)* ] or [RuY2(Pxe2x80x94P)*], in which (Pxe2x80x94P)* is a phosphorated, bidentate chiral ligand, xe2x80x9cdienexe2x80x9d represents a diolefin such as cyclo-octadiene or norbornadiene, Xxe2x88x92 is an anion such as a halide, BFxe2x88x924, PFxe2x88x926 or ClOxe2x88x924, and Y is a halide. The isomerisation reaction gives rise to the corresponding enamines or imines, which are then hydrolysed to obtain chiral aldehydes, for example citronellal, methoxycitronellal or hydroxycitronellal. These compounds are highly valued materials in perfumery.
These known methods are the subject matter of several publications. Of these one may cite the patents EP-B-068 506, 135 392 and 156 607 (holder: Takasago Perfumery Co.) and JP 61-19203 of the same holder, as well as patent EP-B-398 132 (holder: Hoffmann La Roche AG) and patent application EP 643 065 (applicant: Bayer AG). All the methods described in these reference documents use catalysts having, as the ligand, bidentate chiral phosphines based on biphenyl or binaphthyl systems of symmetry C2. The most well-known ligand for this isomerisation reaction, and which is also used in other catalytic methods, is BINAP, represented by the following formula (IV): 
One drawback of the use of the catalysts mentioned above resides in the fact that it is difficult to separate the catalyst from the product obtained. The normal method used for obtaining a pure product is distillation of the reaction mixture, obtained by degrading the catalyst, which is then no longer usable.
The chemical literature proposes the fixation on to polymers of ligands known for their use in catalytic reactions, so as to solve the problems of separation of the product from the reaction mixture and of decomposition of the catalyst still present in this mixture. A general account of the existing knowledge in this field may be found in the article of M. Reggelin in Nach. Chem. Tech. Lab. 45(1997), pages 1196-1201.
Application WO 98/12202 of Oxford Asymmetry Limited describes ligands of the BINAP type such as those mentioned above, which are attached to suitable polymers such as, for example, polystyrene, polyamides or aminomethylated polystyrene, by spacer groups comprising, for example, xe2x80x94Oxe2x80x94C(O)xe2x80x94, xe2x80x94NHxe2x80x94C(O)xe2x80x94, xe2x80x94Oxe2x80x94 or xe2x80x94NHxe2x80x94 functions linked to alkyl chains. In the examples, this same application also describes the use of these ligands immobilised for the asymmetric hydrogenation of ketones and olefins.
Although the data reveal that these ligands yield roughly the same conversions and enantioselectivities as non-immobilised BINAP, nowhere can a precise indication be found of the nature of the products obtained and of the reaction conditions. Overall, the application reveals that immobilised ligands of the BINAP type, which no longer have the C2 symmetry of non-immobilised BINAP, can be used in asymmetrical hydrogenations. However, and also due to the fact that not all the necessary information permitting estimation of the true catalytic capacity of these ligands is available, the person skilled in the art is not capable of deducing that ligands of the immobilised BINAP type prove equally effective in asymmetric isomerisation reactions of allylamines.
The aim of the present invention is to find an immobilised ligand which avoids the problems of non-immobilised ligands as stated above, and which is just as effective in respect of conversions and enantioselectivity as the latter in the asymmetric isomerisation of prochiral allylamines.
The aim is achieved by a method for stereospecific isomerisation of prochiral allyl systems represented by the formula 
in which R1xe2x89xa0R2, and each represents an alkyl or alkenyl group, containing 1 to 12 carbon atoms, or an aryl group, possibly substituted by a hydroxy group, R3 and R4 representing independently of one another hydrogen, a C1 to C12 alkyl or alkenyl group, or an aryl group, R5 is hydrogen or an alkyl or cycloalkyl group, containing 1 to 8 carbon atoms, R6 is an alkyl or cycloalkyl group, containing 1 to 8 carbon atoms, or R5 and R6 are taken together with the nitrogen to form a ring having 5 or 6 atoms, or a ring having 6 atoms and containing oxygen, into enamines represented by the formula 
in which the symbols R1-R6 have the meanings assigned above, or into imines represented by the formula 
in which the symbols R1-R4 and R6 have the meanings assigned above and R5 is hydrogen, by means of catalysts of Rh, Ir or Ru having at least one chiral phosphorated ligand, the method being characterised in that the phosphorated ligand in the catalyst of Rh, Ir or Ru is a ligand represented by the formula 
in which Ar represents a phenyl or tolyl group,
R1 is a unit of the type Zxe2x80x94Bxe2x80x94, in which
B is a group linking the polymer Z with the ligand, selected from among the groups xe2x80x94Oxe2x80x94C(O)xe2x80x94(CH2)nxe2x80x94, xe2x80x94NHxe2x80x94C(O)xe2x80x94(CH2)nxe2x80x94, xe2x80x94Oxe2x80x94(CH2)nxe2x80x94, xe2x80x94NHxe2x80x94(CH2)nxe2x80x94 and xe2x80x94(CH2)nxe2x80x94, n being a whole number from 1 to 10, and
Z is a polymer or copolymer selected from among silica, polystyrene, the polyamides, TENTAGEL resins (grafted copolymers having a low crosslinked polystyrene matrix on which polyethyleneglycol or polyoxyethylene is grafted), the functionalised polystyrene of the Merrifield resin type, aminomethylated polystyrene, or [4-(hydroxymethyl)phenoxymethyl] polystyrene (xe2x80x9cWang resinxe2x80x9d); or in which the phosphorated ligand is a ligand represented by formula 
xe2x80x83in which Ar is a phenyl or tolyl group, R2xe2x89xa0R3 and
R2 and R3 represent hydrogen or a unit of the type Zxe2x80x94Bxe2x80x2xe2x80x94,
Bxe2x80x2 being a group linking the polymer Z and the ligand, selected from among the groups xe2x80x94C(O)xe2x80x94, xe2x80x94(CH2)mxe2x80x94 or xe2x80x94NHxe2x80x94C(O)xe2x80x94(CH2)mxe2x80x94C(O)xe2x80x94, m being a whole number from 1 to 4, and
Z being a polymer as defined above, R2=R3=H being excluded.
We found that the ligands according to formulae (V) and (VI) are very useful for preparing metallic catalysts which are active in the stereospecific isomerisation of allylamines into amines or chiral imines. More particularly, they proved to be highly effective in the isomerisation of diethyl geranylamine and of diethyl nerylamine. We were able to verify that the immobilisation of the ligands used in the present invention has no negative effect on their performance, and even enables advantageous and unanticipated results to be obtained by comparison with non-immobilised ligands.
The polymers or copolymers which may be used as carriers for the ligands in the context of the present invention are the state-of-art ones and include silica, polyamides and polystyrene, particularly functionalised and, as the case may be, cross-linked polystyrenes. Non-limiting examples of this type of polymer are functionalised polystyrenes cross-linked to divinyl benzenes (called Merrifield resins), TENTAGEL resins (grafted copolymers having a low crosslinked polystyrene matrix on which polyethyleneglycol or polyoxyethylene is grafted), functionalised polystyrene of the Wang resin type, or [4-(hydroxymethyl)phenoxymethyl] polystyrene, and aminomethylated polystyrene.
The preferred ligands of formula (V) are those in which R1 is a unit of the type xe2x80x94(CH2)3xe2x80x94C(O)Oxe2x80x94CH2xe2x80x94Z, Z being the resin of the xe2x80x9cWangxe2x80x9d type, or a unit of the type xe2x80x94(CH2)3xe2x80x94C(O)NHxe2x80x94CH2xe2x80x94Z, Z being polystyrene.
Regarding formula (VI) ligands, those in which R2 is a unit of the type xe2x80x94(CH2)2xe2x80x94(Oxe2x80x94C2H4)xxe2x80x94Z or xe2x80x94C(O)xe2x80x94(CH2)2xe2x80x94C(O)xe2x80x94NHxe2x80x94(CH2)2xe2x80x94(Oxe2x80x94C2H4)xxe2x80x94Z are preferred, in which x is a whole number of about 60 and Z is polystyrene. These are then compounds of the formula (VI) in which the active ligand is linked to a resin of the so-called xe2x80x9cTentagelxe2x80x9d type, from the oxygen atom.
Formula (VI) ligands, which constitute another object of the present invention, are new chemical compounds. They are synthesised from 6,6xe2x80x2-bis(diphenylphosphino)biphenyl-2,2xe2x80x2-diol (VII) [obtained by cleavage of (6,6xe2x80x2-dimethoxybiphenyl-2,2xe2x80x2-diyl)-bis(diphenyl phosphine), see EP-A-398 132], by reacting with the desired polymer or a derivative thereof, as shown in the following reaction scheme: 
In the above scheme, Ar, Z and Bxe2x80x2 have the meanings assigned above and M is a suitable leaving group enabling the diol (VII) to be coupled to the polymer used, or to a derivative thereof. Suitable M groups are known to the person skilled in the art, and non-limiting examples may be found below. The choice of M group of course depends on many factors such as, for example, the reactivity of the linking group Bxe2x80x2, and the person skilled in the art is able to select leaving groups M in accordance with his or her chemical knowledge.
Complexes used as metallic complexes which may serve as a catalyst for the isomerisation reaction are those of Rh, Ir and Ru, preferably Rh complexes.
The principle of synthesis of active complexes of rhodium is known. This synthesis involves reacting chiral phosphines of the above formulae with a suitable precursor complex of Rh(I). As examples of the latter we cite complexes of the type [Rh(ene)2Y]2 or [Rh diene Y]2, which react with the phosphorated ligands in the presence of a silver salt of the formula AgX, or complexes of the type [Rh(diene)2]X, which also react with phosphorated ligands, giving rise to catalytically active complexes according to the invention. If required, synthesis of the active complexes is carried out after treatment with hydrogen, at pressures which may be as high as 100 bars, preferably up to 40 bars. The active complexes themselves may be described by the general formulae [RhL*(ene)2]+Xxe2x88x92, [RhL*diene]+Xxe2x88x92, [RhL*]+Xxe2x88x92 or [RhL*2]+Xxe2x88x92. In these formulae, L* indicates a phosphorated chiral ligand as defined by the formulae (V), (IX) or (X) defined above, ene indicates an olefin such as for example ethylene, propylene or butene, diene indicates a diene such as for example 1,3-butadiene, 1,3-pentadiene, cyclopentadiene, 1,5-hexadiene, 1,4-cyclohexadiene, 1,4- or 1,5-heptadiene, 1,4- or 1,5-cycloheptadiene, 1,4- or 1,5-octadiene, 1,4- or 1,5-cyclo-octadiene, or norbornadiene. The preferred dienes are 1,5-cyclo-octadiene (hereinafter abbreviated to xe2x80x9cCODxe2x80x9d) and norbornadiene (hereinafter abbreviated to xe2x80x9cNBDxe2x80x9d). Xxe2x88x92 indicates an anion such as a halide, ClO4xe2x88x92, BF4xe2x88x92, B(C6H5)4xe2x88x92, PF6xe2x88x92, PCl6xe2x88x92, CH3COOxe2x88x92, CF3COOxe2x88x92, SbF6xe2x88x92, AsF6xe2x88x92, CH3SO3xe2x88x92, FSO3xe2x88x92 or CF3SO3xe2x88x92, and Y indicates a bridging anion selected from among the halides.
We found that when CF3SO3xe2x88x92, common name triflate, was used as a counter-ion in the process of the invention, advantageous results were obtained. Not only did the use of this ion frequently lead to the highest conversions and enantiomeric excesses (ee), by comparison with the ions described and used to date in the type of isomerisation which is the object of the present application; it also allowed a reduction in the proportion of the catalyst relative to the substrates. This effect will be more evident from reading the examples presented below.
The complexes of iridium which can be used in the process of the invention are synthesised in way similar to those of rhodium, from a phosphorated ligand and a suitable precursor complex. As an example of the latter we hereby cite the formulae [Ir(COD)Y]2, IrY3, [IrY6]2xe2x88x92 in which COD signifies 1,5-cyclo-octadiene and Y signifies a bridging anion which is a halide. As regards Ru, several known complexes of this metal lend themselves to use in the process of the invention as a precursor complex. By way of a non-limiting example, only the species [Ru2(ACOO)4(H2O)(diene)2] (A=non-substituted alkyl or aryl group or halogen), [RuY2(aryl)]2 (aryl=aryl group such as benzene, toluene, cymene, Y=bridging anion selected from among the halides).
The person skilled in the art is familiar with a large number of prochiral allylamines that may be used as the substrate in the process of the invention and are designated by the formula (I). We therefore refer here to the examples of suitable allylamines cited in patent EP-B-068 506, page 4, lines 1-7, or the prochiral allylamines respectively. The amines cited in this prior art are hereby included by reference.
The most valued substrates for isomerisation according to the invention are diethyl nerylamine and diethyl geranylamine. As other preferred substrates of the present invention, we cite here cyclohexyl geranylamine, methylcyclohexyl geranylamine and (E)- and (Z)-N,N-diethyl-7-hydroxy-3,7-dimethyl 2-octenylamine [see K. Tani, Pure Appl. Chem. 1985, (57), 1845 and J. Am. Chem. Soc. 1984 (106) 5208]. After hydrolysis of the chiral enamine which forms during the process, the process of the invention yields optically active citronellal.
The process of the invention enables allylamines to be isomerised with conversions which may reach 100% and enantiomeric excesses of more than 80%, often approaching 100%. The result depends on the complex and the phosphine used, and also on the reaction conditions, such as the reaction time, temperature, and the amount of catalyst relative to the substrate, etc. The person skilled in the art is capable of adjusting these parameters so as to optimise the reaction yield.
When the method of the invention is performed, the active complex may be synthesised in advance and then added to the reactor, or it may be produced in situ from the precursor complex, such as one described earlier, and the selected chiral phosphine.
The amount of catalyst relative to the substrate may vary from 0.05 mol % to 20 mol %. The catalyst will preferably be used in a proportion of 0.1 to 5 mol %, relative to the substrate. A particular advantage of the use of immobilised ligands such as those described in the present application resides in the fact that separation of the reaction mixture from the catalyst is particularly easy, as it can be done by simple decantation or filtration without the need for distillation. Following separation from the reaction mixture, the catalyst may be reused in the isomerisation reaction, and only very slight deactivation of the catalyst is observed even after several repetitions of the method with the same catalyst.
If necessary, a suitable quantity of the precursor complex is added after a certain number of repetitions, as release of the metal into the reaction medium is occasionally observed. We obtained the best results by using an alcohol, such as ethanol, an ester, such as ethyl acetate, or tetrahydrofuran (THF) as the solvent. These solvents are preferred according to the invention, the best results having been obtained with THF.
The isomerisation reaction may be performed at a temperature between 0xc2x0 C. and 150xc2x0 C., preferably between 40 and 110xc2x0 C. The temperature may be selected as a function of the substrate and of the solvent used, this being within the means of the person skilled in the art.